Photolithography is utilized in the fabrication of semiconductor devices to transfer a pattern onto a wafer. Based on various integrated circuit (IC) layouts, patterns are transferred from a photomask (or a reticles) to a surface of the wafer. As dimensions decrease and density in IC chips increases, resolution enhancement techniques, such as optical proximity correction (OPC), off-axis illumination (OAI), double dipole lithography (DDL) and phase-shift mask (PSM), are developed to improve depth of focus (DOF) and therefore to achieve a better pattern transfer onto the wafer.